With years of development, organic light-emitting diode (OLED) displays have now penetrated into wide applications, ranging from small-medium displays for smart watches and mobile phones, large TVs, to futuristic flexible and foldable displays. However, despite stringent power consumption requirements in many applications, over years OLED displays still suffer unsatisfactory energy efficiency due to light extraction problems inherent in many light-emitting devices. Approaches have been reported for enhancing light out-coupling of OLEDs, but due to difficulties associated with display image quality, fabrication complexity, and integration compatibility, they in general are not applicable for light extraction of OLED displays.
Active-matrix organic light-emitting diode displays (AMOLEDs) have become one of the major display technologies, due to their various attractive merits for display applications. These include excellent viewing characteristics resulting from their self-emissive nature, vivid and saturated colors, high contrast, fast response, wide operation temperatures, and compatibility with flexible and wearable applications etc. For many AMOLED applications (e.g., mobile and wearable applications), the power consumption is a critical issue and thus achieving the highest possible external quantum efficiency (EQE) of pixel OLEDs is essential. The EQE of an OLED is governed by both the internal quantum efficiency (IQE) of charge-to-photon conversion in the device and the extraction efficiency of internally generated light for external viewing. Substantial progresses have been made in developing emitting materials with high IQE, such as phosphorescent and other triplet-harvesting emitters that can give nearly ideal 100% IQE. However, current AMOLEDs still suffer poor light extraction and far from ideal EQEs.
Approaches have been reported for enhancing light out-coupling of OLEDs, such as microlens arrays, surface textures, scattering media, embedded low-index grids, embedded photonic nanostructures (e.g., grating/corrugation/photonic crystals), and high-index substrates etc. Although these different methods/structures may be useful for OLED lighting and/or bottom-emitting OLED structures, they in general are not readily applicable for light extraction of (top-emitting) AMOLED displays, mainly due to several difficulties associated with display image quality, fabrication complexity, and integration compatibility: (1) the out-coupling structures/effects often lead to leakage/diffusion of pixel emission to neighboring pixels, resulting in pixel blurring that would degrade the display resolution and image quality; (2) the out-coupling structures/effects often cause scattering, diffusive and diffractive optical reflection of incident ambient light and thus degrade display contrast and image quality; (3) the optical out-coupling structures may require advanced and expensive fabrication (e.g., high-resolution nano-fabrication) not so compatible with OLED display structures or manufacturing; (4) furthermore, the extraction enhancement offered by these methods/structures is generally still very limited and may be strongly wavelength and viewing-angle sensitive, not desirable for displays. Due to these difficulties, to date AMOLEDs hardly adopt any effective light out-coupling techniques/structures for boosting efficiencies and power saving, although it is highly desired. Here this invention discloses a general, highly effective and scalable extraction-enhancing OLED display pixel structure based on embedding the pixel OLED inside a three-dimensional (3D) optically reflective concave structure selectively filled with a high-index filler material. Ultimately high light extraction efficiency approaching ˜80% is achievable with such a three-dimensional pixel configuration.
Thus to date, a highly effective and feasible light extraction technique/structure that can boost efficiencies and power saving of OLED displays and yet keep image quality is still lacking and remains a grand challenge.